This invention relates to an imprinting apparatus and, in particular, to a non-impact imprinting apparatus for reproducing characters and graphic images by transferring thermoplastic magnetic ink onto a transfer medium by means of heat and magnetism.
A number of miniature and low-cost non-impact imprinting apparatuses using magnetic ink have been proposed. For example, Japanese Patent Publication No. 96541 discloses an imprinting apparatus in which magnetic ink is used as an ink material for magnetothermal transfer of the melted ink. Magnetic attraction force produced by a magnet that is separate from the heat supply acts on the ink in order to form corresponding thermal images. This type of apparatus is illustrated in FIG. 1 of the drawings in the present specification.
In the apparatus of FIG. 1, a thermal head 301 and a magnet 306 are provided. A transfer medium such as transfer paper 305 and an ink medium 302 are adapted to be drawn between thermal head 301 and magnet 306 in such a way that ink medium 302 is positioned adjacent thermal head 301 and transfer paper 305 is positioned adjacent magnet 306. Ink medium 302 consists of base film 303 on the side of thermal head 301 and thermoplastic magnetic ink 304 on the side of and in contact with transfer paper 305. Ink medium 302 and transfer paper 305 are drawn between thermal head 301 and magnet 306 and heat is applied to ink 304 through base film 303 by thermal head 301. This causes melted ink 304 to adhere to transfer paper 305.
If it is desired to permanently adhere an ink dot 307 to transfer paper 305, magnetic force is applied to ink 304 through transfer paper 305. Ink 304 is then torn off base film 303 and preferentially adheres to transfer paper 305. If no permanent adherence is desired, no magnetic force is applied and ink 304 is separated from transfer paper 305 and remains preferentially adhered to base film 303 of ink medium 302.
In this apparatus, ink 304 contacts both base film 303 and transfer medium 305. This is true independent of whether ink 304 is at a recording portion and is to be transferred or at a non-recording portion and is not to be transferred. Problems arise because once the ink at a recording portion is melted and stuck onto transfer paper 305, base film 303 can be torn off together with ink 304, resulting in defective transfer. Additionally, ink 304 can remain adhered to transfer paper 305 at a portion where ink transfer is not desired.
Referring to FIG. 2, thermal transfer recording can generally be accomplished under the condition of inequality, that is:
F.sub.B, F.sub.A &lt;&lt;F.sub.C, F.sub.D
wherein F.sub.A and F.sub.B are forces for accelerating the transfer of recording portion ink onto transfer paper and in which F.sub.A represents the adhesive force between the ink and the transfer paper and F.sub.B represents the cohesive force within the recording portion ink; and wherein F.sub.C and F.sub.D are forces for inhibiting the transfer of the ink and in which F.sub.C represents the adhesive force between the ink and the base film and F.sub.D represents the cohesive force between recording portion ink and non-recording portion ink. As shown in FIG. 2, which includes a base film 311, a recording portion ink 312, a non-recording portion ink 313 and a transfer paper 314, F.sub.A and F.sub.B are directed downwards toward transfer paper 314 and away from base film 311 and F.sub.C and F.sub.D are directed upwards away from transfer paper 314 and towards base film 311.
The melted recording portion ink 312 is attracted toward transfer paper 314 by magnetic attraction forces F.sub.A and F.sub.B so that the probability of adherence of melted ink 312 to transfer paper 314 is increased as the magnetic attraction forces F.sub.A and F.sub.B are increased. The efficiency of transfer can be improved by increasing the force F.sub.A. However, base film 311 also adheres to ink 312 at the time of adherence of ink 312 to transfer paper 314. Consequently, forces F.sub.C and F.sub.D directed toward base film 311 also exist. When transfer paper having greatly inferior surface smoothness is used, magnetic attraction force F.sub.A may become less than either of transfer inhibiting forces F.sub.C or F.sub.D, thereby causing defective transfer.
As shown in FIG. 3, prior art printing methods cannot provide normally formed recording dots when transfer paper 322 has inferior surface smoothness. This is due to the existence of non-contact portions, i.e. valley portions 324, between transfer paper 322 and magnetic ink 323. Magnetic ink 323 sticks to the convex portion 325 of the fiber top of the surface of the paper and does not adhere on valley portions 324. Accordingly, non-uniform recording dots 323 are provided. In particular, normally formed recording dots cannot be obtained when rough paper having Beck's smoothness equal to 1-2 sec. is used.
A top plan view of a recorded dot 332 on rough paper is shown in FIG. 4. As shown, the magnetic ink sticks only to the convex portion 331 at the top of the fiber and not in the valley portions. Accordingly, a non-uniform recorded dot 332 is formed.
The same phenomenon occurs in the case of high resolution recording dots. In particular, normally formed recording dots cannot be obtained when recording dots having small areas are desired.
According to the conventional method described, thermoplastic magnetic ink 304 is in contact with transfer paper 35, as shown in FIG. 1. Accordingly, much of the heat produced by thermal head 301 passes through magnetic ink 304 and escapes in transfer paper 305. This creates an efficiency problem due to the fact that a large portion of the heat is not used to melt the thermoplastic magnetic ink and is lost as heat at the time of transfer.
Additionally, friction, heat conduction and the like arise between thermoplastic magnetic ink 304 and transfer paper 305. This creates problems because the thermoplastic magnetic ink can adhere to a non-recording portion of the transfer paper by means other than normal magnetic recording means. This phenomenon is known as "print stain".
Accordingly, it is desirable to provide an imprinting apparatus for reproducing characters and grahic images by transferring thermoplastic magnetic ink onto a transfer medium by means of heat and magnetism that overcomes the disadvantages of prior art imprinting apparatus.